1. Field of the Invention
The present invention relates to two-axis measurement systems, and more particularly, to a robotic scanner for performing planar near-field antenna measurements which includes a control system to maintain thermal stability of the scanner in an otherwise uncontrolled thermal environment.
2. Description of Related Art
High performance antennas are increasingly prevalent in the art as spacecraft, aircraft, ship and ground vehicle mission requirements become more sophisticated. One problem in the development and manufacture of high performance antennas is the measurement of antenna performance. Traditionally, antenna measurement was conducted by placing the antenna at a remote location to measure the amplitude response characteristics of the antenna in its operational range. Typical operational distances for high gain antennas can range from fifty feet to three miles. This measurement technique, known as far-field testing, suffers from significant limitations, such as susceptibility to weather effects, ground reflections and increasing real estate costs.
As an alternative to far-field testing, near-field testing was developed. A nearfield test is conducted in an indoor test range using a microwave probe to sample the field radiated near the antenna under test (AUT). A computer collects the amplitude and phase data sampled by the microwave probe, and calculates the far-field equivalent response using a Fourier transform technique. Accurate near-field measurements require that all the significant antenna energy be sampled by the microwave probe. Highly directive antennas, such as reflectors and waveguide phased arrays, beam most of the energy in the forward direction normal to the antenna aperture. To test these types of antennas, a planar near-field robotic scanner is utilized to move the microwave probe along a planar pattern approximately normal to the antenna aperture. To accurately reconstruct the measured field, the probe must sample the antenna energy at a plurality of points with a minimum spacing based on the Nyquist sampling theorem. A near-field measurement system of this nature is described in U.S. Pat. No. 5,408,318 to Slater, for WIDE RANGE STRAIGHTNESS MEASURING SYSTEM USING A POLARIZED MULTIPLEXED INTERFEROMETER AND CENTERED SHIFT MEASUREMENT OF BEAM POLARIZATION COMPONENTS, assigned to the same assignee as the present invention, the subject matter of which is incorporated herein by reference.
The near-field measurement technique is also applicable to other types of reflecting bodies, emitters/receptors or transducers having other types of emitted waveforms, such as optical or acoustic waves, and is effective in measuring performance of antennas, lenses, anechoic chambers and compact ranges. The measuring probe may act as both a transmitting antenna and a receiving antenna for measuring a reflected phase front from a reflecting body. The reflecting or transducing bodies discussed herein are collectively referred to as antennas or transducers.
To make accurate near-field measurements, all the significant antenna energy must be sampled by the probe. Highly directive antennas, such as reflectors and waveguide phased arrays, send most of the energy in the forward direction normal to the antenna aperture. To test these types of antennas, a planar near-field scanner is utilized. Precision positioning systems, such as Cartesian robots, are used to move the probe along a planar raster pattern approximately normal to the antenna aperture. Cylindrical and spherical scanners are also possible, in which the AUT is rotated relative to a measuring probe. To accurately reconstruct the measured field, the probe must sample points at some minimum spacing which is usually less than half the wavelength of the antenna signal (.lambda./2). Therefore, to achieve an accurate near-field measurement, the precise position of the probe and its planarity with respect to the AUT is critical.
Obtaining high accuracy position information for the probe relative to the test article has proven to be difficult to achieve. Undesired variations in the spacing between the probe and the AUT can be experienced due to thermally induced expansion and contraction of the scanner structure. Even in a controlled environment, such as an indoor testing facility, thermal variations may be experienced due to periodic cycling of the air conditioning system within the facility. The temperature fluctuations affect the near-field measurement by distorting the shape of the AUT mount and near-field scanner mount that adversely affects azimuth and elevation of the AUT with respect to the probe. These distortions in AUT position further result in distortion of the energy emitted by the AUT, and decrease the accuracy of the near-field measurement. A method of correcting for thermal drift between the AUT and the scanner is disclosed in U.S. Pat. No. 5,419,631, to Slater, for THREE-AXIS MOTION TRACKING INTERFEROMETER FOR MEASUREMENT AND CORRECTION OF POSITIONAL ERRORS BETWEEN AN ARTICLE UNDER TEST AND A MEASUREMENT PROBE, assigned to the same assignee as the present invention, the subject matter of which is incorporated herein by reference. As with the near-field scanner structure, the phase reference cable that carries the signal from the probe is highly susceptible to temperature changes, which can cause physical changes in the length of the phase reference cable by as much as several wavelengths. Such dimensional changes adversely affect the accuracy of the near-field measurement.
It is increasingly desirable to perform some antenna near-field testing at a field location of the antenna, without removing the antenna to an indoor test facility. For example, certain large antennas mounted on ships cannot be easily removed from service for periodic testing, and it would therefore be advantageous to perform near-field tests on these antennas while in their operational location. The outdoor position of these antennas presents a particularly harsh thermal environment, with unpredictable variations in temperature of the AUT and the scanner due to atmospheric effects, as well as thermal coupling of the scanner to local heat sources or sinks, uneven heat transfer rates, and material mismatches.
Thus, a critical need exists to increase the thermal stability of the scanner to permit operation in an uncontrolled environment. It would be particularly desirable to provide a thermally stable scanner without substantially increasing its operational weight or complexity.